Most edible liquids, such as juices from vegetables, fruits, grains and grasses, contain an undesirable microbial content. In the past such liquids have been sterilized or sanitized by means of extreme heat for sustained periods of time, sometimes under pressure, and then cooled. Such heating is done in bulk or after placing in containers, such as cans or bottles. High temperatures can change the chemical composition of such fluids and can also negatively affect the quality and taste of the liquid being processed.
High Pressure Processing (HPP), or Ultra High Pressure (UHP) is a method that has been developed wherein fluids are subjected to elevated pressures, i.e. from about 30,000 psi to 150,000 psi, for a period of time with or without being heated, to minimize, kill or inactivate microbial organisms. HPP inactivates most vegetative microorganisms, including bacteria, yeasts and molds, while allowing treated fluids to retain food quality, freshness and have an extended shelf life. Some bacterial spores can also be treated. HPP processing enables treated fluids to retain the fresh characteristic of food without degradation that is usually caused by thermal treating. In fact, HPP can be conducted under ambient or lower temperature conditions. In that regard HPP is especially suited to treatment of fluids that are subject to thermal degradation even at minimally elevated temperatures.
In the past high temperature processing has been considered essential to kill microbes such as bacteria, viruses, spores, and other single cell pathogens that cause spoilage in foods and food borne disease. However high temperature processing often results in off flavors, protein denaturization, and other nutritional problems.
Moreover, HPP exerts a uniform pressure throughout the pressure device. It is thought to have minimal effect on low molecular weight compounds such as flavors, vitamins, pigments, and the like when compared to thermal treatments.
In general, in an HPP process, the product to be treated is usually in a flexible container, i.e. a plastic bag or bottle and is loaded into a high pressure chamber or container which contains a pressure-transmitting or hydraulic fluid, typically water. The chamber is pressurized by a pump raising the pressure of the pressure-transmitting fluid to the desired range, e.g. 30,000 to 150,000 psi for a predetermined period of time, usually about two to eight minutes. The pressure is transmitted uniformly to the product being treated. The pressure is removed and the product being treated is taken from the chamber for packaging, bottling, or like means and is ready for distribution and/or storage under primarily ambient conditions.
As a net result HPP results in a treated product that has an extended shelf life at ambient or, in some cases, refrigerated temperatures. For example, high acid products, i.e. having a pH of less than about 6.0 may be stored at ambient temperatures whereas some low acid foods such as milk, vegetables, soups and the like, i.e. having a pH of greater than about 7.5 may require some refrigeration to attain an extended shelf life.